everyone loves a tourney, but I've done 6inch battles before in my Fleet for Fleet Events and eventually with the belt a bb5 has room for, you'd never actually be able to kill anyone, no matter how much ap/heavy you had. With BB3s I've done it with a 15 inch limit, and it would still take all day. Then we had also tested 22 inches on a Bismarck and it was actually impossible to kill with 6inch guns.
Just a heads up, we had no lag (but that can differ for anyone), the guns capped reload pretty reasonably, and on the BB3s there was room for about 70+ binds (depending on ship).
Gotta say though, I imagine if you did this with BB2s (as much as everyone would prefer their BB3s) it would probably be pretty great too; BB3s like that were a blast! Like a high-level Blitz! (strongly suggest no torps/HH...)